feat!: scope chat model override settings to organizations (#28442)

Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

coderd/coderd.go

@@ -1436,6 +1436,23 @@ func New(options *Options) *API {
 			httpmw.ExtractOrganizationParam(options.Database),
 		).Get("/chats/models", api.listChatModelConfigsByOrganization)
 
+		r.Route("/organizations/{organization}/chats/model-overrides", func(r chi.Router) {
+			r.Use(
+				apiKeyMiddleware,
+				httpmw.ExtractOrganizationParam(options.Database),
+			)
+			r.Get("/", api.getOrganizationChatModelOverrides)
+			r.Put("/{context}", api.putOrganizationChatModelOverride)
+		})
+		r.Route("/organizations/{organization}/members/{user}/chats/model-overrides", func(r chi.Router) {
+			r.Use(
+				apiKeyMiddleware,
+				httpmw.ExtractOrganizationParam(options.Database),
+				httpmw.ExtractOrganizationMemberParam(options.Database),
+			)
+			r.Get("/", api.getUserChatPersonalModelOverrides)
+			r.Put("/{context}", api.putUserChatPersonalModelOverride)
+		})
 		r.Route("/organizations/{organization}/chats/models", func(r chi.Router) {
 			r.Use(apiKeyMiddleware)
 			r.With(httpmw.ExtractOrganizationParam(options.Database)).Get("/", api.listChatModelConfigsByOrganization)
@@ -1473,12 +1490,8 @@ func New(options *Options) *API {
 				r.Put("/system-prompt", api.putChatSystemPrompt)
 				r.Get("/plan-mode-instructions", api.getChatPlanModeInstructions)
 				r.Put("/plan-mode-instructions", api.putChatPlanModeInstructions)
-				r.Get("/model-override/{context}", api.getChatModelOverride)
-				r.Put("/model-override/{context}", api.putChatModelOverride)
 				r.Get("/personal-model-overrides", api.getChatPersonalModelOverridesAdminSettings)
 				r.Put("/personal-model-overrides", api.putChatPersonalModelOverridesAdminSettings)
-				r.Get("/user-personal-model-overrides", api.getUserChatPersonalModelOverrides)
-				r.Put("/user-personal-model-overrides/{context}", api.putUserChatPersonalModelOverride)
 				r.Group(func(r chi.Router) {
 					r.Use(httpmw.RequireExperimentWithDevBypass(api.Experiments, codersdk.ExperimentChatVirtualDesktop))
 					r.Get("/computer-use-provider", api.getChatComputerUseProvider)